Agrawal Sraddha, Lin Wei, Prezhdo Oleg V, Trivedi Dhara J
Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350108, People's Republic of China.
J Chem Phys. 2020 Aug 7;153(5):054701. doi: 10.1063/5.0010628.
Graphitic carbon nitride (g-CN), a metal-free and visible light responsive photocatalyst, has garnered much attention due to its wide range of applications. In order to elucidate the role of dimensionality on the properties of photo-generated charge carriers, we apply nonadiabatic (NA) molecular dynamics combined with time-domain density functional theory to investigate nonradiative relaxation of hot electrons and holes, and electron-hole recombination in monolayer and bulk g-CN. The nonradiative charge recombination occurs on a nanosecond timescale and is faster in bulk than the nanosheet, in agreement with the experiment. The difference arises due to the smaller energy gap and participation of additional vibrations in the bulk system. The long carrier lifetimes are favored by small NA coupling and rapid phonon-induced loss of quantum coherence between the excited and ground electronic states. Decoherence is fast because g-CN is soft and undergoes large scale vibrations. The NA coupling is small since electrons and holes are localized on different atoms, and the electron-hole overlap is relatively small. Phonon-driven relaxation of hot electrons and holes takes 100-200 fs and is slightly slower at higher initial energies due to participation of fewer vibrational modes. This feature of two-dimensional g-CN contrasts traditional three-dimensional semiconductors, which exhibit faster relaxation at higher energies due to larger density of states, and can be used to extract hot carriers to perform useful functions. The ab initio quantum dynamics simulations present a comprehensive picture of the photo-induced charge carrier dynamics in g-CN, guiding design of photovoltaic and photocatalytic devices.
石墨相氮化碳（g-CN）是一种无金属且对可见光有响应的光催化剂，因其广泛的应用而备受关注。为了阐明维度对光生载流子性质的作用，我们应用非绝热（NA）分子动力学结合时域密度泛函理论来研究单层和体相g-CN中热电子和空穴的非辐射弛豫以及电子-空穴复合。非辐射电荷复合发生在纳秒时间尺度上，且体相中的复合速度比纳米片更快，这与实验结果一致。这种差异是由于体相系统中较小的能隙以及额外振动的参与。较小的NA耦合以及激发态和基态电子态之间声子诱导的量子相干快速丧失有利于长载流子寿命。退相干很快，因为g-CN较软且会发生大规模振动。NA耦合较小，因为电子和空穴定域在不同原子上，且电子-空穴重叠相对较小。热电子和空穴的声子驱动弛豫需要100 - 200飞秒，并且由于参与的振动模式较少，在较高初始能量下弛豫速度会稍慢一些。二维g-CN的这一特性与传统三维半导体形成对比，传统三维半导体由于更大的态密度在较高能量下表现出更快的弛豫，并且可用于提取热载流子以执行有用功能。从头算量子动力学模拟呈现了g-CN中光诱导载流子动力学的全面图景，为光伏和光催化器件的设计提供了指导。
